The purpose of the catalyst stripper of the FCC unit is to remove hydrocarbons adsorbed on the interior of spent catalyst before the regeneration step. Basically, the FCC catalyst stripper is a moving fluidized bed with counter-current contacting between steam and hydrocarbon-rich catalyst. The majority of the stripping steam normally flows upward in the form of bubbles. Hydrocarbons are adsorbed on the surface of the catalyst pores and migrate through the pores and later through the interstitial spaces between particles.
The stripping process is basically a mass transfer process, being normally carried out in a fluidized bed (dense phase), where the catalyst or “emulsion” made up of catalyst and hydrocarbons counter-currently meets a gas flux, normally steam, that strips hydrocarbons occluded in the catalyst. This way, stripping is essentially a gas-solid extraction. Depending on the operational conditions, some cracking, dealkylation and condensation reactions may also occur. Strippable hydrocarbons not desorbed in the stripping step are directed to the regenerator, causing product waste. Besides, the higher the hydrogen/carbon ratio in the hydrocarbon mixture carried to the regenerator, higher are the resulting regeneration temperatures. High regeneration temperatures result in deep catalytic deactivation and limit catalyst circulation, leading to losses in conversion and valuable product yields. According to the practiced temperature levels, even the stripper itself may be jeopardized in view of the metallurgical limit.
The efficiency of the catalyst stripper has a significant impact on the overall FCC unit profitability. Poor stripping reduces the amount of recoverable product. Perhaps, more importantly, poor strippping can cause excessively high regenerator temperatures, which reduce catalyst circulation, unit severity and conversion. Higher regenerator temperatures also lead to faster catalyst deactivation and increased mechanical wear on the unit. Industrial experience has shown that improving a poorly operating FCC catalyst stripper can increase a refiner's FCC unit conversion by 2 volume % and overall profits by about US$ 0.15 per barrel of feed.
Improved stripper efficiency is directly transferred to the unit operation, reducing the entrainment of high Hydrogen/Carbon ratio hydrocarbons to the regenerator and consequently, reducing the delta coke and the heat produced during coke burning. Upon decreasing temperature of the regenerator dense phase, it is possible to obtain higher catalyst circulation, leading to higher conversion or allowing increased flow rate of processed feed.
Besides, the negative impact of the increase in regeneration temperature on catalyst deactivation should be mentioned. High temperatures increase vanadium mobility and promote the formation of acidic species, which attack the catalyst structure, leading to permanent activity loss.
As regards the stripper role, it should be mentioned that any modification introduced in the unit aiming at reducing delta coke, such as for example, new feed-dispersion technologies, might not attain the desired goals if the stripper cannot withstand the corresponding circulation increase. This way, a low efficiency stripper may reduce the potential benefits of new technologies.
Stripper performance is a function of several operational variables and design parameters. However, obtaining optimum-stripping performances intrinsically depends on the adequate selection of internals used in the stripping apparatus. Internals modification is especially important in already existing units, those being limited by the feed flow rate and having maximum circulation.
In order to improve steam-catalyst contacting and increasing stripping efficiency, the strippers of an industrial FCC unit contain internals. The most commonly used internal devices are baffles. Since optimization of stripper internals is the most interesting alternative to attain best stripper performance, and in view of the low efficiency of these devices, efforts are being endeavored to increase the efficiency of it.
U.S. Pat. No. 3,728,239 teaches an improved process of gas-solid contact when treating finely divided solids with a gas in a vertical cylindrical vessel. The vessel is provided with a plurality of vertically spaced, downwardly sloping baffles, which preferably overlap one another, and with a substantially vertical impingement plate mounted directly above a downwardly sloping baffle. It is alleged that stripping efficiency is substantially improved if the catalyst mass is alternately compacted and expanded while surrounded by the stripping medium. The suggested apparatus is particularly suitable for stripping residual hydrocarbons from catalyst used in fluid catalytic cracking processes.
In order to improve stripping efficiency, U.S. Pat. No. 4,721,603 teaches, for a unit devoid of closed cyclones, modifications in the upper part of the stripper, such modifications involving baffles and being able to lead to a partial cyclone effect so as to improve stripping efficiency.
U.S. Pat. No. 5,910,240 teaches improved designs for the stripper section of an FCC unit. The improved stripper section of the said US patent contains a plurality of vanes used to impart rotational movement to the FCC catalyst system as it traverses through the stripper section vertically by way of gravity and horizontally (radially) by way of the slanted surfaces of the stripper trays. According to said document, the improved tray design adds a third dimension to the flow of catalyst through the stripper section by providing rotational flow means on the surface of the slant trays to impart a rotational flow, i.e., angular motion, to the catalyst as it traverses the stripper section. The other two dimensions are the vertical flow of the catalyst through the stripper, due to gravity, and the horizontal or radial flow of the catalyst due to the slanted nature of the tray. The angular/rotational movement of the spent catalyst improves catalyst/steam contacting and improves stripping efficiency. In the proposed configuration, one of the advantages of the concept of U.S. Pat. No. 5,910,240 is that a large stripper surface is open, allowing the passage of pieces of refractory and/or coke from the separator vessel.
U.S. Pat. No. 5,531,884 teaches a fluidized catalytic cracking process (FCC) and apparatus using a catalyst stripper with slant trays or shed trays having “downcomers”. Downcomers are vertical catalyst/gas contacting elements that provide a vertical, counter current region for catalyst/stripping vapor contact. It is alleged that downcomers improve stripping effectiveness by preserving the static head of pressure existing under the tray.
U.S. Patent Application Publication 2002/0008052A1 teaches a baffle-style stripper for an FCC process having a complete or nearly complete coverage of stripping openings over the sloped surface of the baffle that will provide improved stripping efficiency and catalyst flux through the stripper. The alleged advantage is to prevent choking of stripper flow by the restriction of stripping gas flow to narrow open areas between the sloped baffles. The small (3.8 cm) openings are able to produce a small penetration jet in the catalyst descending from the top of the baffle. As it is believed that the mass transfer occurs in the region next to the jet and in the bubbles formed by the jet, the higher efficiency results from the small bubbles formed by the small jets. Cold-flow test results provided in said US application confirm improved efficiency of the so-modified baffles as compared to downcomers of the above cited U.S. Pat. No. 5,531,884 for a wide range of catalyst flux, chiefly for high catalyst fluxes of the order of 683,480 kg/h/m2.
U.S. Pat. No. 5,549,814 teaches the use of a grid arrangement that provides increased contacting of stripping fluid and catalyst through multiple levels of stripping grids while using a configuration that permits access through the stripper vessel for maintenance and inspection. The stripper grids also have orifice openings to redistribute stripping fluid at each level of stripping grid and increase contact between catalyst and stripping fluid. The distance between the grids is in the range of 24″-48″ so as to obtain the desired efficiency. One drawback of the concept of this U.S. patent stems from the fact that not having free areas may cause that coke pieces or any other debris may be stuck in the stripper.
U.S. Pat. No. 5,716,585 teaches the use of various packing elements that occupy the whole stripping section, that make possible the radial distribution of catalyst and steam, so as to improve stripping efficiency.
More recently, U.S. Pat. No. 6,244,833 teaches the use of a structured packing, this increasing the effective volume of the stripper, the residence time and consequently the stripping efficiency. According to said patent, a gas-solid fluidized bed is formed within a contacting element having pairs of planar portions arranged in intersecting planes, each planar portion being formed by one or more webs and one or more open slots adjacent each web, the webs and slots being arranged such that a web in one of the planar portions intersects a slot in the paired planar portion. The fluidized bed is made up of catalyst particles fluidized by a gas stream, such as in a catalyst stripper and/or regenerator in a FCC system. The structured packing that is suggested in this US patent leads to the same drawbacks as those of small open area that can be easily blocked by refractory and coke pieces or debris.
A further approach to improving stripping efficiency is taught by European publication EP 0187032A1, whereby stripping is made in two steps, with a first conventional step using baffles and a second step where a portion of the regenerated hot catalyst is admixed to the catalyst effluent from the first stripping step, leading to a temperature around 80° C. higher than the temperature of the riser top. A higher temperature would favor hydrocarbon desorption from the catalyst surface.
Aiming at minimizing the use of stripping steam as well as increasing the overall process efficiency, U.S. Pat. Nos. 4,927,606 and 5,015,363 teach steam distribution throughout several injection points, situated below each baffle. This device eliminates stagnated areas and secures a well-distributed, counter current flow to the catalyst steam flow. Experimental data indicate that the proposed device allows a reduction from 1.5 kg steam/catalyst ton to 0.7 kg steam/catalyst ton, keeping constant the regenerator dense phase temperature.
State-of-the-art documents indicate that in known strippers, that is, disc and donut devices, optimized steam-catalyst contact is not favored at the desired extent. The area above the baffle is a stagnated zone where hydrocarbons desorption from catalyst is of low efficiency. These devices favor bubble coalescence, forming large bubbles, those large bubbles disturbing the fluid dynamics, this in turn rendering flow and steam-catalyst contact more difficult.
Normally, to increase stripper efficiency, one can increase the stripper steam rate using an efficient steam distributor as well as increase the steam rate. However, even if an efficient steam distributor (pipe-grid) is used, this cannot be of much help because of the above-mentioned coalescence effect. Due to backpressure, the increased steam rate on its turn increases steam entrainment, which can limit the maximum possible steam rate.
Therefore, in spite of the technological progress in this field, the technique still needs improvements in the spatial arrangement of the stripper baffles, that is, segmented, parallel baffles arranged in sequence, the segment number as well as the plate thickness being dimensioned so as to reduce the coalescence of the bubble size of the stripping fluid as well as an homogeneous fluid dynamic distribution of the catalyst flux, turbulence throughout the stripping vessel being secured, said arrangement in a stripper apparatus of an FCC unit being described and claimed in the present application.